Abstract:

In July and August 1996, a combined physical oceanography and acoustics experiment was performed in the Mid-Atlantic Bight south of Nantucket Shoals. The purpose of this experiment was to examine coastal frontal variability and its impact on acoustic propagation and imaging. Extensive oceanographic measurements of the experimental region were made. These measurements included: (1) SeaSoar hydrography; (2) moored T, S, and current meter sensors; and (3) satellite infrared imaging of the sea surface temperature. These measurements provided what is believed to be the highest resolution imaging of a shelfbreak frontal region yet obtained. Acoustic measurements in the experimental region included: (1) a three-dimensional tomographic imaging array; and (2) aircraft deployed SUS drop fields. Observations revealed the presence of a strong jet associated with sharp thermal gradients. During this time period, the front was contorted by the presence of a streamer of shelf water moving offshore, which resulted in large thermal gradients in both along the shelf and across the shelf direction. The oceanographic activity produced significant mode coupling in the acoustic signals, leading to large time spreading of the source pulses.